Add hack for using qcow2 ephemeral base images

Base images are currently using the raw disk format, rather than
the qcow2 format, which is wasting about 350G of disk space on
all compute nodes.

This change also changes the default openstack version from essex
to folsom.

Change-Id: I832b3d9ffd430748604b723f9718b4229ae21906
Merge "Add hack for using qcow2 ephemeral base images" into production